Ignition device with precombustion chamber coated with a refractory coating layer, for internal combustion engine, and igniter with precombustion chamber

ABSTRACT

The invention concerns an ignition device for internal combustion engine, containing: 
         a main chamber ( 1 ) designed for including a main combustible mixture, and fitted with a compression system of said mixture, and    an igniter ( 11 ) containing a precombustion chamber ( 2 ) designed for receiving reactants and an ignition system ( 13,14 ) of the reactants contained in the precombustion chamber ( 2 ), said precombustion chamber ( 2 ) being defined by a precombustion chamber body ( 12 ) having a head ( 12   a ) including at least one passageway ( 15 ), said head ( 12   a ) of the precombustion chamber body separating the precombustion chamber ( 2 ) from the main chamber ( 1 ) and communicating the precombustion chamber ( 2 ) and the main chamber ( 1 ) by dint of the passageway(s) ( 15 ), 
 
characterised in that the head ( 12   a ) is coated at least partially externally with a coating layer (R) of at least one refractory material.

The present invention concerns an ignition device for internalcombustion engine, as well as an igniter with precombustion chamber.

The ignition device according to the invention comprises an igniter withprecombustion chamber which may be screwed instead of a conventionalignition sparking plug without any modifications of the cylinder head ofthe internal combustion engine (diameter smaller than or equal to 14mm), the means for igniting an oxidant and fuel mixture being containedin a precombustion chamber defined by a body whereof the head is fittedwith passageways.

Thus, when the igniter with precombustion chamber is mounted in thecylinder head of the engine, the precombustion chamber of the igniter isseparate from the main combustion chamber of the engine by the head ofthe precombustion chamber body and communicates with the main combustionchamber by dint of the passageways provided in such head.

The igniter with precombustion chamber may possibly be fitted with meansenabling to introduce directly the reactants in the precombustionchamber.

The patent U.S. Pat. No. 4,926,818 describes a device and a method forgenerating pulsed jets designed to form swirling combustion pockets. Thedevice described comprises a main chamber containing a main combustiblemixture wherein a piston travels and a precombustion chamber receivingreactants and communicating with the main chamber via orifices drilledin a wall. The ignition of the reactants in the precombustion chambergenerates gas jets in combustion, which ignite the main mixturecontained in the main chamber by convection of the flame front.

The patent application FR 2 781 840 describes an ignition device ofinternal combustion engine containing:

-   -   a main chamber designed for including a main combustible        mixture, and fitted with a compression system of said mixture,    -   a precombustion chamber designed for receiving reactants and        communicating with the main chamber via orifices drilled in a        wall separating the main chamber of the precombustion chamber,    -   a system for igniting the reactants contained in the        precombustion chamber.

In such a device, which proves globally satisfactory, the orifices areof small diameter and capable of preventing the propagation of a flamefront while enabling the propagation of the unstable compounds derivedfrom the combustion of the reactants contained in the precombustionchamber. The compression system and the seeding of the main mixture withunstable compounds enable mass self-ignition of the initial mixture.

The patent application FR 2 810 692 also concerns an ignition device forinternal combustion engine including a precombustion chamber generallycylindrical in shape, similar to that described in the application FR 2781 840, but whereof the passageways communicating with the maincombustion chamber are circumscribed by a circular curve running throughthe centres of the outermost passageways, the diameter of such circularcurve being in a ratio smaller than or equal to ½ with the diameter ofthe cylindrical precombustion chamber. Such an arrangement enables theoperation of the engine with a small quantity of oxidant air, inparticular when the composition of the air-fuel mixture in the mainchamber is stoichiometric, for depollution purposes with a three-waycatalyst.

These devices may still be improved.

Notably, the present invention concerns an ignition device for internalcombustion engine which may exhibit the following advantages:

-   -   increased efficiency of the combustion in the precombustion        chamber,    -   improved operation of the engine on small loads, and enhanced        stability of the operation of the engine in idle mode.

To this end, the invention concerns an ignition device for internalcombustion engine, containing:

-   -   a main chamber designed for including a main combustible        mixture, and fitted with a compression system of said mixture,    -   an igniter containing a precombustion chamber designed for        receiving reactants and an ignition system of the reactants        contained in the precombustion chamber, said precombustion        chamber being defined by a precombustion chamber body having a        head including at least one passageway, said head of the        precombustion chamber body separating the precombustion chamber        from the main chamber and communicating the precombustion        chamber and the main chamber by dint of the passageway(s).

According to the invention, the head is coated at least partiallyexternally with a coating layer of at least one refractory material.

By refractory material is meant a non-metal material, but which maycontain a metal constituent, whereof the pyroscopic resistance isequivalent to 1500° C.

Preferably, the precombustion chamber body is coated at least partiallyinternally with a coating layer of at least one refractory material.

Preferably still, the passageway(s) are coated with a coating layer ofat least one refractory material.

According to a preferred embodiment of the invention, the coating layeris a nano-structured coating layer, i.e. whereof the size of the grainsis greater than or equal to 1 nm and smaller than 1000 nm.

The refractory coating layer according to the invention may consist of asingle layer of at least one refractory material or of two layers of atleast one refractory material.

The refractory material(s) may be any type of refractory materials usedcurrently to resist high temperatures and which are capable of resistingpressure constraints due to the operation of the ignition device.

One may quote in particular nitrides, borides, silicides, carbides,zirconium alloys, yttrium alloys, titanium alloys and boron alloys,oxides, preferably aluminium, titanium, iron, silicium, cerium,manganese and zirconium oxides, as well as zirconias having beensubjected to the addition of at least one metal oxide selected amongcalcium, magnesium, yttrium, hafnium and rare earth oxides.

By rare earth is meant all the elements whereof the atomic number rangesbetween 57 (lanthanum) and 71 (lutetium).

Preferably, the refractory material(s) are selected among Al₂O₃, SiO₂,CeO₂, MnO₂, ZrO₂, ZrY, Zr and Y being in stoichiometric proportions ornot, and TiB₂, preferably among Al₂O₃, ZrY, Zr and Y being instoichiometric proportions or not, and TiB₂.

Preferably, the thickness of the coating layer of the refractorymaterial(s) ranges between 0.5 and 100 μm, preferably still between 1and 50 μm.

The application of the refractory coating layer according to theinvention may take place by a liquid phase method, the liquid phasehaving suitable wetting and surface tension properties.

The deposition of the refractory coating layer according to theinvention may still take place by the use of powder, by thermalprojection (by electric arc or by plasma), by chemical vapour phasedeposition (CVD), by physical vapour phase deposition (PVD) or still byelectro-deposition.

The coating layer of at least one refractory material according to theinvention enables to provide a thermal barrier effect.

Notably, the refractory coating layer on the external face of the headof the precombustion chamber body enables to limit the thermal transfersbetween the main mixture in combustion and the body of the precombustionchamber.

The refractory coating layer on the internal wall of the precombustionchamber body enables to limit the thermal transfers between thereactants in combustion in the precombustion chamber and the body of theprecombustion chamber.

The limitation of the thermal transfers towards the body of theprecombustion chamber improves the combustion in the precombustionchamber accordingly.

The refractory coating layer according to the invention is stilladvantageously hooked correctly to the substrate represented by the bodyof the precombustion chamber, which is generally a copper alloy.

Moreover, the refractory coating layer according to the inventionenables to reduce or to prevent the adhesion of the compounds derivedfrom the combustion of the reactants in the precombustion chamber, onthe walls of the body of precombustion chamber, of the head or of thepassageways.

In the case of a two-layer coating, the function of non-adhesion of thecompounds derived from the combustion is fulfilled by the upper layer,and the thermal barrier function is fulfilled by the lower layer.

Finally, the small thickness and the structure of the refractory coatinglayer according to the invention enables to avoid too high differentialexpansion constraints.

Thus, the operation of the engine on small loads and the stability inidle modes are improved.

Such refractory coating layers are particularly suited to precombustionchamber igniters intended for use with heavily supercharged internalcombustion engines, i.e. having an Average Effective Pressure greaterthan or equal to 13 bars.

According to a first embodiment, the ignition of the main mixturecontained in the main chamber takes place by convection of the flamefront derived from the ignition of the reactants contained in theprecombustion chamber.

In such a case, the passageway(s) are preferably of cylindrical shapeand of diameter greater than 1 mm.

According to a second embodiment, the passageway(s) are capable ofpreventing the propagation of a flame front while enabling thepropagation of unstable compounds derived from the combustion of thereactants contained in the precombustion chamber, the compression systemof the main chamber and the seeding of the main mixture with saidunstable compounds enabling mass self-ignition of the main mixture.

The self-ignition in a large volume enables very quick pressure rise, areduced pinkling and good repeatability.

In such a case, said passageway(s) are preferably of cylindrical shapeand of diameter smaller than or equal to 1 mm.

Preferably still, said passageway(s) have a length smaller than or equalto the diameter thereof. By length is meant the dimension of thepassageways according to a direction perpendicular to the surface of theseparation wall. This way, the smallest possible quantity of unstablecompounds are trapped to the walls.

Generally, the number of passageway(s) ranges between 1 and 20,preferably between 3 and 15.

In the case of self-ignition of the mixture by seeding of the mainmixture with unstable compounds, according to a preferred embodiment:

-   -   the upper section of the body of precombustion chamber, not        adjoining the main chamber, is in the form of a cylinder of        inner diameter φ, and    -   the head of the precombustion chamber body comprises several        passageways, said passageways being circumscribed by a circular        curve of diameter d₂ running through the centres of the        outermost passageways, the ratio d₂/φ being smaller than or        equal to 0.5.

Preferably, the ratio d₂/φ is smaller than or equal to ⅓.

Advantageously, the centre of the curve running through the centres ofthe outermost passageways is situated on the axis symmetry of theprecombustion chamber.

But according to another embodiment, the centre of the curve runningthrough the centres of the outermost passageways may be situated at adistance d₃ from the axis of symmetry of the precombustion chamber,equal to or greater than the quarter diameter φ of the precombustionchamber. Such configuration enables to direct preferably the jets offlames or of unstable compounds towards a particular zone of thecombustion chamber, in relation to the position of said centre of thecurve with respect to the axis symmetry of the precombustion chamber.

The invention still concerns an igniter for internal combustion enginecontaining a precombustion chamber defined by a precombustion chamberbody having a head fitted with at least one passageway, theprecombustion chamber being designed for including a combustiblemixture, and an ignition system of the combustible mixture contained inthe precombustion chamber, the head being coated at least partiallyexternally with a coating layer of at least one refractory material.

Preferably, the precombustion chamber body is coated at least partiallyinternally with a coating layer of at least one refractory material.

The passageway(s) of the head may also be coated with a coating layer ofat least one refractory material.

Preferably, the refractory material(s) are selected among nitrides,borides, silicides, carbides, zirconium alloys, yttrium alloys, titaniumalloys and boron alloys, oxides, preferably aluminium, titanium, iron,silicium, cerium, manganese and zirconium oxides, and zirconias havingbeen subjected to the addition of at least one metal oxide selectedamong calcium, magnesium, yttrium, hafnium and rare earth oxides.

Preferred refractory materials are for instance Al₂O₃, SiO₂, CeO₂, MnO₂,ZrO₂, ZrY, Zr and Y being in stoichiometric proportions or not, andTiB₂, preferably Al₂O₃, ZrY, Zr and Y being in stoichiometricproportions or not, and TiB₂.

Preferably, the thickness of the coating layer ranges between 0.5 and100 μm, preferably between 1 and 50 μm.

The invention will be understood better and other aims, advantages andfeatures thereof will appear more clearly when reading the followingdescription, in conjunction with the appended drawings.

FIG. 1 represents a schematic, partially sectional view, of an ignitiondevice including an igniter with precombustion chamber according to theinvention.

FIG. 2 represents an enlargement of FIG. 1.

FIG. 3 represents a schematic, vertically sectional view of theprecombustion chamber body of an igniter according to the invention.

FIG. 4 is a view from beneath of the head of a precombustion chamberbody of an igniter according to the invention.

A cylinder of an internal combustion engine, represented on FIG. 1,includes a main chamber 1 delineated by a jacket (not represented) andclosed at the upper section thereof by a cylinder head 10. As usual, themain chamber 1 contains a piston (not represented) actuated intranslation by a rod (not represented).

An igniter 11 with precombustion chamber according to the invention isattached in the cylinder head 10 in order to be adjoining the mainchamber 1, for instance by screwing in a thread 10 a of the cylinderhead 10.

The igniter 11 includes a precombustion chamber body 12, generallytubular in shape, containing a head 12 a, preferably having the form ofa spherical cap, defining a precombustion chamber 2.

The head 12 a of the precombustion chamber body 12 forms a separationwall between the main chamber 1 and the precombustion chamber 2. Thehead 12 a communicates the precombustion chamber 2 with the main chamber1 by dint of passageways 15.

According to the embodiment represented on FIGS. 1, 2 and 3, theinternal wall of the precombustion chamber body 12, the external wall ofthe head 12 a and the walls of the passageways 15 are coated with acoating layer (R) of at least one refractory material.

Generally, the precombustion chamber 2 has a volume ranging between 0.2cm³ and 2 cm³, preferably ranging between 0.5 cm³ and 1.5 cm³.

Generally, the ratio SN between the sum of the sections of thepassageways 15 of the precombustion chamber and the volume of theprecombustion chamber ranges between 10⁻³ mm⁻¹ and 5.10⁻² mm⁻¹.

Optionally, the igniter may moreover include an intake (not represented)enabling to supply the precombustion chamber 2 with a mixture ofair-fuel reactants formed upstream or to introduce fuel, the air beingmixed with fuel in the precombustion chamber 2.

The precombustion chamber is fitted with an ignition system containing acentral electrode 13 and a ground electrode 14.

When the ignition of the main mixture takes place by convection of theflame front from the precombustion chamber, the passageways 15 havepreferably a diameter greater than 1 mm.

To prevent, at ignition, the propagation of a flame front while enablingthe propagation of unstable compounds (ignition of the main mixture byself-ignition), the passageways 15 have then a small diameter, generallysmaller than 1 mm, and, advantageously, a length smaller than thediameter thereof.

In the case of self-ignition of the main mixture, as shown on FIG. 2,the passageways 15 belong advantageously to a circle of diameter d₂corresponding substantially to half the diameter φ of the precombustionchamber.

The centre of this circle may be on the axis symmetry 2 b of theprecombustion chamber 2, as shown on FIG. 3.

The centre of this circle may also be situated at a distance d₃ from theaxis of symmetry 2 b of the precombustion chamber 2, as shown on FIG. 4,whereon passageways 15, 8 in number, have been represented.

One injects an air-fuel mixture in the main chamber and one supplies theprecombustion chamber 2. One then produces a spark between theelectrodes 13 and 14 while triggering thus the combustion in theprecombustion chamber 2, so that the temperature and the pressureincrease therein.

Under the effect of the higher pressure in the precombustion chamber 2than in the main chamber 1, the flames, or the unstable compounds in thecase when the dimension of passageways prevents the propagation of theflame front, are expelled in the form of jets towards the main chamber1. Thus the main mixture contained in the main chamber 1 is ignited.

In both cases (ignition of the main mixture by convection of the flameis front or by self-ignition), the refractory coating layer on theinternal wall of the precombustion chamber body 12, on the external wallof the head 12 a and on the walls passageways 15 limits the heattransfers from the gases in combustion towards the precombustion chamberbody 12, which increases considerably the efficiency of combustionaccordingly in the precombustion chamber 2.

One improves thus the operation of the engine on small loads and at idlemode.

1. An ignition device for internal combustion engine, containing: a mainchamber designed for including a main combustible mixture, and fittedwith a compression system of said mixture, and an igniter containing aprecombustion chamber designed for receiving reactants and an ignitionsystem of the reactants contained in the precombustion chamber, saidprecombustion chamber being defined by a precombustion chamber bodyhaving a head (12 a) including at least one passageway, said head of theprecombustion chamber body separating the precombustion chamber from themain chamber and communicating the precombustion chamber and the mainchamber by dint of the passageway(s), characterised in that the head iscoated at least partially externally with a coating layer (R) of atleast one refractory material.
 2. Ignition device according to claim 1,wherein the precombustion chamber body is coated at least partiallyinternally with a coating layer (R) of at least one refractory material.3. An ignition device according to claim 1 wherein the passageway(s) arecoated with a coating layer (R) of at least one refractory material. 4.An ignition device according to claim 1 wherein the coating layer (R) isa nano-structured coating layer, the size of the grains being greaterthan or equal to 1 nm and smaller than 1000 nm.
 5. An ignition deviceaccording to claim 1, wherein the coating layer (R) consists either of alayer of at least one refractory material, or of two layers of at leastone refractory material.
 6. An ignition device according to claim 1,wherein the refractory material(s) are selected among nitrides, borides,silicides, carbides, zirconium alloys, yttrium alloys, titanium alloysand boron alloys, oxides, preferably aluminium, titanium, iron,silicium, cerium, manganese and zirconium oxides, and zirconias havingbeen subjected to the addition of at least one metal oxide selectedamong calcium, magnesium, yttrium, hafnium and rare earth oxides.
 7. Anignition device according to claim 1 wherein the refractory material(s)are selected among Al₂O₃, SiO₂, CeO₂, MnO₂, ZrO₂, ZrY, Zr and Y being instoichiometric proportions or not, and TiB₂, preferably among Al₂O₃,ZrY, Zr and Y being in stoichiometric proportions or not, and TiB₂. 8.An ignition device according to claim 1, wherein the thickness of thecoating layer ranges between 0.5 and 100 μm, preferably between 1 and 50μm.
 9. An ignition device according to claim 1, wherein thepassageway(s) are of cylindrical shape and of diameter greater than 1mm.
 10. An ignition device according claim 1, wherein the passageway(s)are capable of preventing the propagation of a flame front whileenabling the propagation of unstable compounds derived from thecombustion of the reactants contained in the precombustion chamber, thecompression system of the main chamber and the seeding of the mainmixture with said unstable compounds enabling mass self-ignition of themain mixture.
 11. An ignition device according to claim 10, wherein saidpassageway(s) are in the form of a cylinder of diameter smaller than orequal to 1 mm.
 12. An ignition device according to claim 10, whereinsaid passageway(s) have a length smaller than or equal to the diameterthereof.
 13. An ignition device according to claim 10, wherein: theupper section of the precombustion chamber body is in the form of acylinder of inner diameter Φ, and the head of the precombustion chamberbody comprises several passageways, said passageways being circumscribedby a circular curve of diameter d₂ running through the centres of theoutermost passageways, the ratio d₂/Φ being smaller than or equal to0.5.
 14. An ignition device according to claim 13, wherein the ratiod₂/Φ is smaller than or equal to ⅓.
 15. An ignition device according toclaim 13, wherein the centre of the curve running through the centres ofthe outermost passageways is situated on the axis symmetry of theprecombustion chamber.
 16. An ignition device according to claim 13,wherein the centre of the curve running through the centres of theoutermost passageways is situated at a distance d₃ from the axissymmetry of the precombustion chamber, said distance d₃ being equal toor greater than the quarter diameter Φ of the precombustion chamber. 17.An igniter for internal combustion engine containing a precombustionchamber defined by a precombustion chamber body having a head fittedwith at least one passageway, the precombustion chamber being designedfor including a combustible mixture, and an ignition system of thecombustible mixture contained in the precombustion chamber,characterised in that the head is coated at least partially externallywith a coating layer (R) of at least one refractory material.
 18. Anigniter according to claim 17, wherein the precombustion chamber body iscoated at least partially internally with a coating layer (R) of atleast one refractory material.
 19. An igniter according to claim 17,wherein the passageway(s) are coated with a coating layer of at leastone refractory material.
 20. An igniter according to claim 17, whereinthe refractory material(s) are selected among nitrides, borides,silicides, carbides, zirconium alloys, yttrium alloys, titanium alloysand boron alloys, oxides, preferably aluminium, titanium, iron,silicium, cerium, manganese and zirconium oxides, and zirconias havingbeen subjected to the addition of at least one metal oxide selectedamong calcium, magnesium; yttrium, hafnium and rare earth oxides.
 21. Anigniter according to claim 20, wherein the refractory material(s) areselected among Al₂O₃, SiO₂, CeO₂, MnO₂, ZrO₂, ZrY, Zr and Y being instoichiometric proportions or not, and TiB₂ preferably among Al₂O₃, ZrY,Zr and Y being in stoichiometric proportions or not, and TiB₂.
 22. Anigniter according to claim 17, wherein the thickness of the coatinglayer ranges between 0.5 and 100 μm, preferably between 1 and 50 μm.